The invention relates to a circuit arrangement of an anti-theft alarm for a vehicle which, after an activation of the alarm by signal generator, intermittently emits optical alarm signals in cycles by means of light sources provided on the vehicle. The alarm is controlled by a control unit that contains a separate clock generator for clocking the light sources via lines, in the event of an alarm.
An anti-theft alarm, which usually generates acoustic alarm signals by means of a horn or the like, can be coupled to the hazard warning system of a vehicle so that additionally, intermittent optical alarm signals are generated by means of the vehicle indicator lights. The vehicle issuing the alarm can thus be better located, especially in darkness.
An anti-theft alarm which operates in a corresponding manner is known from German Patent Document DE 38 10 806 Cl. The control unit contains a clock generator which, for an alarm, intermittently operates all the vehicle indicator lights via a control unit double contact relay that is switched over for a limited time. It is not disclosed in this German Patent Document how this device interacts with the normal hazard warning system. The publication also contains no information relating to the relationship of the flashing frequencies in the hazard warning mode and in the alarm mode of the indicator lights, and does not contain information on how the relationship could be adjusted.
One problem in this respect is that specific countries require different flashing speeds for the hazard warning mode and for the alarm mode. This serves above all to distinguish between break-down situations in which the hazard warning system has to be switched on manually and the above described alarm cases in which unauthorized attempts at access are being signalled. In the European Community, doubling the frequency of the hazard warning mode is prescribed for the case of an alarm. In Japan, a single frequency is admissible. Also, Switzerland uses a continuous audible tone while the European community uses intermittent alarm tones.
The alarm state is usully maintained for a limited amount of time (max. 5 minutes) and is continuously retriggered during unauthorized driving. For example, actuating the brakelight switch each time the brakes are actuated or actuating other electrical loads in the vehicle can retrigger the alarm. However, stolen vehicles which are driven with the alarm system activated should be roadworthy. In other words, it must still be possible for the normal driving direction indicator signals and hazard warning signals to be generated. Under certain circumstances, the indicator lights activated by the anti-theft alarm could light up with an undefined rhythm when the normal flashing system is switched on because of two different load-dependent flasher generators whose load-dependent function is disturbed by the externally supplied voltage levels. Other light sources, which, depending on countries' regulations, may also be operated by means of an anti-theft alarm, for example headlights and rear lights, should also be capable of being switched on normally even during unauthorized driving.
A combination driving direction indicator and hazard warning device for motor vehicles is described in German Patent Document DE 36 30 448 Cl. A driving direction indicator signal can be randomly superimposed on the hazard warning mode by the actuation of the driving direction indicator switch, this hazard warning mode having been randomly switched on, for example, when a vehicle is being towed away.
An interrelation with an anti-theft alarm which can also trigger a (modified) hazard warning mode, is not disclosed in the aforesaid publication. In some vehicles, a microprocessor-controlled flasher generator (time function relay) is used to provide different flashing frequencies. With the aid of a special control input on the flasher generator connected to the control unit of the anti-theft alarm, a control signal issued by the unit in the event of an alarm can be appropriately evaluated and can trigger the alarm flashing mode at increased frequency, while in hazard warning mode the usual, series-produced hazard warning switch in the vehicle is used.
Furthermore, a separate clock relay is used in this system on each side of the vehicle for the respective indicator lights so that the superimposition of driving direction indicator signals, already mentioned, is made possible by means of hazard warning signals. The anti-theft control unit here does not contain a separate clock generator, but instead is realized by means of a time function relay.
However, for reasons of cost and assembly it would be advantageous to be able to use a conventional hazard warning system with a simple load-controlled clock generator with a relay for the application outlined above together with an anti-theft alarm without having to provide a special anti-theft input. It is known in conventional systems of this kind to switch the hazard warning mode by connecting all the indicator lights on both sides of the vehicle simultaneously to the clock generator via the hazard warning switch. By means of such a measure, preliminary operations for possible installation of an anti-theft alarm as an optional extra during manufacture of the vehicle can be dispensed with.
The invention has as an object to provide a circuit for an anti-theft alarm in a manner so that even when the alarm is triggered, it is possible to drive in a manner which is safe in relation to the safety of other motorists and with respect to the optical signals emitted.
According to the present invention, a circuit arrangement of an anti-theft alarm for a vehicle has light sources which are activatable via the circuit arrangement and via externally supplied electrical signals. The circuit arranement has a control unit with a clock generator for intermittently and cyclically activating the light sources via lines upon detection of an alarm and with control logic for detecting an externally supplied electrical signal occurring on the lines and for suppressing the cyclical activating of the light sources with the clock generator for a predetermined amount of time.
Although the vehicle light sources provided for this purpose are cyclically clocked by the anti-theft alarm control unit in the event of an alarm, at least in the dark phase of the clock cycles the control unit monitors the lines to the light sources which can also be activated via other switches, in particular the driving direction indicator switch or the hazard warning switch. In the simplest manner (plausibility test), it can be concluded by detecting a positive voltage level occurring in the dark phase of each alarm flashing cycle on a line to the light sources that the voltage level is being externally supplied and thus one of the other switches mentioned has been closed.
If, on the other hand, the lines are continuously monitored, that is to say also in the light phase, by making a comparison between the time of the signal occurring and an alarm-conditioned clocking, which is usually switched by a relay, self-inhibiting of the alarm flashing is excluded. This can be realized by logical connection of electrical signals.
As soon as an externally supplied signal is present, the anti-theft alarm control unit withdraws from the light source circuit for a predetermined time period so that in the interest of safety of other motorists, the other switched-on switch is given priority. In the customary case of optional alarm clocking of the driving direction indicator lights, opposing clocking of two different clock generators is reliably prevented without additional intervention into the driving direction indicator clock generator present being required. The clock generator can thus be of very simple design while the necessary circuit measures can be displaced into the area of the anti-theft alarm which can be supplied as an optional extra.
Of course, other equally valid methods for signal detection can also be applied. For example, a current which flows to the light sources in the dark phase could also indicate externally supplied signals.
With an anti-theft alarm control unit of the type proposed here containing a microprocessor (.mu.P), it is also possible to fulfill the different requirements relating to the flashing frequency of the optical alarm signals without additional hardware. With appropriate programming of the .mu.P, the desired flashing frequency can be stipulated to the microprocessor via a pin set/cable set encoding which is performed externally at the control unit. For example, by means of two terminal pins which optionally remain free or are connected to earth, four different stipulations can be made. At the same time, in the case of possible retrofitting of a vehicle, it is not necessary to carry out any hardware changes in the control unit itself or even to replace it. These features only come to bear in the software of the .mu.P and in an external changing of the code.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.